In dissociative identity disorder (DID), each alter, also known as an alternate personality state, may have unique characteristics, including language abilities and memories. It is possible for an alter to have knowledge of languages that the host (the primary identity) does not.
In the context you described, where the host watches media in a specific language with subtitles and still listens to the original language, it is plausible for an alter to learn and understand the language without the host being aware of it. The alter's experiences and exposure to the language through media could lead to them acquiring knowledge and skills independently from the host.
DID is characterized by the presence of two or more distinct personality states that frequently take control of the individual's behavior. These alternate personality states may have different abilities, preferences, memories, and even unique identities with individual names. As a result, they can have distinct language skills based on their experiences and exposure.
It's important to note that DID is a complex and often misunderstood disorder. The way language abilities and knowledge are distributed among alters can vary significantly from one person to another. The presence of different language abilities in alters is just one example of the diversity that can exist within DID systems.
Therapy and support from mental health professionals specializing in dissociative disorders are crucial for individuals with DID to understand and work with their unique experiences, including language abilities among alters. If you or someone you know is experiencing symptoms of DID or any mental health concerns, seeking professional help is highly recommended.